Ascites is a common complication of cirrhosis and carcinoma. Its presence significantly worsens the prognosis. A high percentage of patients having cirrhosis who do not respond to a salt restricted diet are dead within a year. Mortality in such patients is due to renal failure brought about by diminished renal perfusion secondary to a diminished blood volume (Clairmont, R. J., Gastroenterol., 1967, 53:220-228). The peritoneovenous shunt has been a significant advance in the therapy of this condition. Ascitic fluid has its ultimate origin from blood and should be returned to the circulating blood from where it arose. The shunt drains ascitic fluid from the peritoneal cavity via flexible tubing through a sensitive, one-way, pressure activated valve into a tube whose tip is located in a major thoracic vein. The shunt system thus continuously infuses ascitic fluid into the blood. The descent of the diaphragm during respiration lowers the pressure in the thorax to below atmospheric pressure enabling air to enter the lungs. The force required to pump the fluid is supplied by a differential pressure between the peritoneal fluid and a large intrathoracic vein. This differential pressure is brought about by a negative intrathoracic pressure which occurs during respiration with descent of the diaphragm and a simultaneous increase in intraperitoneal pressure as the descent of the diaphragm decreases the volume of the peritoneal cavity. Once the fluid gains access to the circulating blood, the body excretes any excess fluid as urine. Although excessive amounts of water and salt are excreted by the kidney, the plasma proteins present in the ascitic fluid are retained.
Flow sensitive valves require a small amount of backflow to close the valve orifice. A small amount of backflow into the venous tubing will obstruct it with clots. A normally closed sensitive pressure activated valve is mandatory to prevent regurgitation of blood into the tubing (LeVeen, H. H., Ann. Surg., 1974, 180:580). A normally closed pressure sensitive valve prevents backflow of blood into the venous tubing which would obstruct the lumen with clots. An obstruction to the free flow of ascitic fluid into the venous system results in failure of the shunt. Although the valve must be normally closed, the valve must open at very low pressures (1 to 4 cm of water) since the differential pressures which supply the necessary pumping force are extremely low. The diaphragm of the valve is manufactured of a low durometer biocompatible elastomer such as silicone rubber together with the struts which suspend the diaphragm over the valve's seat. The struts being elastomeric elongate with pressure and allow the diaphragm to rise and open the valve at low pressures.